Leona is boring.
No seriously, she is. Look at her lore and think about it. She simply is, and therefor she is boring.
Diana on the other hand is a rebellion. She forges her own path and does whatever the hell she wants. She has immense character development potential and a long way ahead of her.
But for Leona there is nothing new. The only way she would ever change is through massive outside influence, like Diana, or a war, or something.
So ... -> Diana.
P.s. : Leona's spells aren't as fluid as Diana's. Not just in the form of her spells, but also the way they are carried out in-game. Diana has short cast times, a very versatile skill shot, and so much more!
While Leona stands still when she casts her spells, they are all straight-forward and stiff and extremely pre-determined.
But it's good that way. They are opposites after all. I just wish they could learn from one another.
P.s.s. : What's up with those elders, tho' ?! Executing anything they don't like. Aren't they worried about population or something? Good thing Diana got rid of them ...